Expansion processes have been widely used for hydrocarbon liquids recovery in the gas processing industry for ethane and propane recovery. External refrigeration is normally required in such processes where the feed gas contains significant quantities of propane and heavier components. For example, in a typical turbo-expander plant, the feed gas is cooled and partially condensed by heat exchange with process streams and/or external propane refrigeration. The condensed liquid containing the less volatile components is then separated and fed to a fractionation column which is operated at a lower pressure than the feed gas pressure. The remaining vapor portion is letdown in pressure in a turbo-expander, resulting in further cooling and liquid formation. With the expander discharge pressure typically at demethanizer pressure, the two-phase stream is fed to the demethanizer with the cold liquids acting as the top reflux to absorb the heavier hydrocarbons. The remaining vapor combines with the column overhead as a residue gas, which is then heated and recompressed to pipeline pressure.
However, in many expander plant configurations, the residue vapor from the demethanizer still contains a significant amount of ethane or propane plus hydrocarbons that could be recovered if chilled to a lower temperature, or subjected to a rectification stage. While lower temperature can be achieved with a higher expansion ratio across the turbo-expander, various disadvantages arise. Among other things, higher expansion typically results in lower column pressure and higher residue gas compression horsepower requirements, making high recovery uneconomical. Lower demethanizer pressure is known to be more prone to CO2 freezing problems which limit the ethane recovery level. Therefore, many NGL recovery configurations employ an additional rectification column, and use of a colder and leaner reflux stream to the fractionation column overhead vapor (see below). Furthermore, most known NGL recovery configurations are optimized for a single mode of operation (i.e., ethane recovery or propane recovery). Thus, when such NGL plants are required to switch recovery mode (e.g., from ethane recovery to propane recovery or ethane rejection), the energy efficiency and propane recovery levels tend to significantly drop. Still further, substantial reconfiguration and operation conditions are necessary in most plants to achieve acceptable results. For example, most of the known ethane recovery plants recover more than 98% of propane and heavier hydrocarbons during the ethane recovery, but often fail to maintain the same high propane recovery during ethane rejection. In ethane rejection operation, the propane recovery levels from such processes often drop to about 90% or lower, thereby incurring significant loss in product revenue.
Present NGL recovery systems can be classified into single-column configurations or two-column configurations, and some operating differences are summarized below. A typical single-column configuration for ethane recovery is described in U.S. Pat. No. 4,854,955. Such configuration may be employed for moderate levels of ethane recovery (typically 75%). In such plants, the column overhead vapor is cooled and condensed by an overhead exchanger using refrigeration content of the column overhead. This additional cooling step condenses the ethane and heavier components from the column overhead gas, which is recovered in a downstream separator and returned to the column as reflux. For ethane rejection, this column operates as a deethanizer, and the column pressure is typically about 350 psig to allow for generation of sufficient refrigeration from turbo-expansion and for ethane/propane separation. However, the lower column pressure generally results in an increased residue gas compression horsepower demand. Other NGL recovery configurations that employ a single column for both ethane recovery and ethane rejection are described in U.S. Pat. No. 6,453,698. Here, an intermediate vapor stream is withdrawn from the column that is cooled in order to generate a reflux to the mid section of the column. While the heat integration, reflux configuration, and process complexity vary among many of these designs, all or almost all fail to operate on ethane recovery and ethane rejection mode and require high energy consumption.
Alternatively, a typical two-column NGL plant employs a reflux absorber and a second column that is operated as a demethanizer or deethanizer, which generally allows more flexibility in operating the absorber and the second column at different pressures. However, conventional two-column plants are generally only economic for either ethane recovery or propane recovery, but not both, and switching recovery modes will often incur significant propane losses, typically at less than 90%. In all operations, propane product is a valuable commodity and high recovery at 99% level is desirable.
For example, in U.S. Pat. Nos. 5,953,935 and 5,771,712, the overhead vapor or liquid from the demethanizer is recycled to the upstream absorber as a lean reflux. While such plants provide relatively high ethane and propane recoveries during ethane recovery, ethane rejection with high propane recovery is not achievable without extensive re-configurations. Alternatively, as shown in U.S. Pat. No. 6,363,744, a portion of the residue gas stream from the residue gas compressor discharge is recycled as a lean reflux in the demethanizer. However, using residue gas to generate a cold reflux for the demethanizer is necessary for high ethane recovery (over 90%) but not energy efficient when used for propane recovery or ethane rejection. In other words, the use of the residue gas recycle for chilling is an over-kill for propane recovery. Moreover, almost all of the above configurations require cryogenic operating temperatures for both the absorber and the distillation columns and require excessive energy during ethane rejection when only propane product is required. In another example, high ethane recovery without CO2 freezing problems is described in U.S. Pat. App. No. 2010/0011809. However, such systems typically do not allow for operational flexibility.
In improved configurations and methods, as for example disclosed in U.S. Pat. No. 7,051,553 and WO 2005/045338, flexibility of operation is provided by use of two reflux streams and by changing process temperature and the feed point of one of the two reflux streams into the absorber. While such plant configurations provide at least some operational. flexibility, various drawbacks (e.g., relatively complex configuration) nevertheless remain. The above noted patents and patent applications, as well as all other extrinsic materials discussed herein, are incorporated by reference in their entirety. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
Thus, numerous attempts have been made to improve the efficiency and economy of processes for separating and recovering ethane and heavier natural gas liquids from natural gas. However, all or almost all of them fail to achieve economic operation when ethane rejection is required. Moreover, currently known configurations fail to provide flexibility in operation where recovery of ethane is only temporarily desired. Therefore, there is still a need to provide improved methods and configurations for flexible natural gas liquids recovery.